The present invention concerns an oligosilazane-containing condensation product, a method for its production, a method for production of a substrate coated or infiltrated with a ceramic-like material using this condensation product and the coated or infiltrated substrate obtained according to this method.
The economic loss caused every year by wear and corrosion of materials is estimated at several billion DM. Interest in protective layers for materials that adversely affect the advantageous properties of the materials as little as possible and at the same time guarantee effective protection against environmental effects and wear has continuously risen in parallel with development of new materials. Increasingly often the base material acquires a purely supporting role, whereas the coating fulfills the actual function. Coatings must therefore be developed for all types of substrates, metals, polymers, glass and ceramics. Both new coating methods and materials are required for this, which are geared not only to material requirements, but also economic ones.
Varnishes and paints, metal coatings, or enamel layers have traditionally been used as protective layers. Varnishes and paints are characterized by simple application (brushing, dipping, spraying). However, they exhibit drawbacks in terms of low temperature stability, insufficient wear and corrosion protection, as well as limited resistance to solvents.
Metal coatings are generally applied by means of melting baths by electrochemical deposition or by PVD or CVD processes. Typical examples are metal layers obtained by galvanizing or chrome plating. However, metal coatings generally exhibit limited corrosion stability, especially limited stability relative to acids and alkalis. Moreover, ecological problems arise owing to the fact that the employed chemicals are often toxic and their handling and disposal pose serious problems.
Enamel layers are used as scratch-proof layers for decorative purposes and in the domestic area. The advantage lies in their simple and thus cost-effective application (dipping bath). In many applications the increases in strength and abrasion properties achieved, however, do not meet the requirements to a sufficient degree. Enamel layers are also very sensitive to mechanical deformations.
Because of their high corrosion and temperature stability, their hardness and ecological compatibility, interest in the production of ceramic protective layers has therefore steadily increased over the last 50 years.
Oxide ceramic layers can be-produced by CVD or PVD methods or by wet chemical methods using the so-called sol-gel process. The advantages of the sol-gel process are high purity of the employed educts that can be attained and thus a high purity of the ceramic layer, good homogeneity of the layer, as well as the possibility of also coating internal surfaces, like the insides of pipes. However, in addition to the high cost of the educts, drawbacks include limited attainable layer thickness ( less than 1 xcexcm) and restriction to oxide systems. The emphasis in layers produced in the sol-gel process therefore now lies in the areas of optically and electrically conducting layers, as well as in the decorative field.
Nonoxide ceramic compounds between the elements boron, carbon, nitrogen and silicon or titanium are also characterized by high [degree of] hardness and wear or corrosion resistance. Silicon nitride (Si3N4) is of special interest as a first-rate material for use in engine construction. Because of its high modulus of elasticity, limited heat expansion, and high resistance to elastic deformation, this material can be employed in a variety of ways. Nonoxide ceramic layers, like TiN, TiCN or even amorphous carbon are now applied in industry primarily with the CVD/PVD process. Reproducible hard material layers adapted to special problems can be produced in this way. The mentioned methods are suitable in particular for automatic process control. The drawbacks of these methods are the high equipment expense and the high costs resulting from this. The high temperatures required, difficult handling of the gaseous starting materials and the formation of aggressive byproducts are drawbacks in the CVD process. Precise process control is essential for production of high-quality PVD layers; internal coating with this method can only be accomplished with great difficulty.
For the reasons just mentioned, the search for alternatives to the CVD and PVD methods has intensified in recent years in order to produce nonoxide ceramics, especially with the elements boron, carbon, nitrogen, silicon, and titanium. One approach is the production of these ceramic layers by pyrolysis of appropriate organoelemental polymers. Thus, it is known that polysilazanes, polyborosilazanes or polycarbosilanes can be converted to ceramic materials with the elements Sixe2x80x94N, Bxe2x80x94Si, or Cxe2x80x94Si. Production of ceramics from such organoelemental compounds can be divided into four steps:
synthesis of preceramic oligomers or polymers from monomer units
crosslinking of these precursors to form two- or three-dimensional preceramic networks
conversion of the network to covalent ceramics by transition from an organic to an inorganic phase mostly by heat treatment (xe2x80x9cceramizationxe2x80x9d)
optionally crystallization of the amorphous solid to thermodynamically stable phases via different metastable intermediate states.
High-purity products with a completely homogeneous distribution of elements at the atomic level and controllable microstructures are obtained with this method at relatively low temperatures.
G. Ziegler, J. Hapke, and J. Lucke report in xe2x80x9cCeramic Transactions,xe2x80x9d 58 (1995), pp. 13-22 on an attempt to produce ceramic phases (materials) from precursors obtained by the reaction of Ti(NMe2)4 with low-molecular polysilazane. The vinyl- and methyl-substituted polytitanosilazane PTS2 so obtained is described as a highly viscous resin, the methyl-substituted PTS1 as a solid. The following formula is given for PTS1: xe2x80x94[R1SiR2xe2x80x94NR3xe2x80x94]n, in which R1=H, R2=Me, and R3=Ti(MMe2)3. The formula of PTS2 corresponds to that of PTS1, except for the fact that R1=xe2x80x94CHxe2x95x90CH2. Carbon fibers and silicon carbide fibers are infiltrated under reduced pressure with liquid precursor and heated to as much as 1000xc2x0 C. after curing at about 250xc2x0 C. to produce ceramic materials in this publication. According to the data of the authors the ceramic precursors, for example PTS1, which are formed after cooling of the reaction mixture as solids, can also be used for infiltration if the reaction to PTS1 is run at lower temperatures (about 60xc2x0 C.) and shorter reaction times. However, conversion under these conditions is incomplete and the ceramic precursors, like PTS1, therefore acquire unconverted educts. The authors propose in particular infiltration of fibers with liquid (educt-containing) PTS1 at about 60xc2x0 C. in order to complete the reaction to PTS1 by heating to 110-120xc2x0 C. During additional heat treatments above this temperature the ceramic precursors are then converted to infusable solids or amorphous ceramics.
The two process variants (for PTS1 and PTS2), however, exhibit decisive shortcomings. The highly viscous resins (vinyl-substituted polytitanosilazanes, like PTS2) exhibit poor adhesion to the substrate and run off the substrate at higher temperatures. When educt-containing PTS1 is used, crosslinking of the silazane educt by titanium atoms only starts during pyrolysis and leads to serious swelling of the polymer because of the numerous gaseous cleavage products. Only very inadequate ceramic materials are therefore obtained.
The task of the present invention was therefore to find precursors for ceramic-like materials,
with which any substrate can be properly wetted,
which also exhibit excellent adhesion to the substrate at the temperatures required for curing and ceramization,
which produce a homogeneous, especially crack-free layer, during curing and ceramization, and
which lead to the desired ceramic-like materials in high yield.
A further task of the present invention was to find an appropriate method for application/infiltration of these precursors onto/into the substrate and for curing of the precursors, which leads to homogeneous ceramic-like materials that exhibit no cracks in particular (for example, from decomposition products).
Finally it was a task of the present invention to offer a ceramic-like material (in conjunction with a substrate), which has a homogeneous, especially crack-free structure and is characterized by properties like hardness, wear resistance, temperature and corrosion resistance.
The present invention concerns a synthesis process for an oligosilazane-containing condensation product (curable precursor for ceramic-like material), comprising the following step:
conversion of oligosilazane composition with an average molecular weight of no more than 1000 g/mol,
with a dialk(en)ylamino compound of the following formula (I)
A[N(R4)2]mxe2x80x83xe2x80x83(I)
xe2x80x83in which A represents at least one element chosen from B, Al, Ti, Zr and Hf, R4 is an alkyl group or alkenyl group and m is the valence of element A,
in which the reaction is run so that an average of 1.6-2.2 mol HN(R4)2 per mol of dialk(en)ylamino compounds of formula (I) is split off.
The present invention also concerns an oligosilazane-containing condensation product obtained according to this process.
Finally, the present invention concerns a process for production of a substrate coated or infiltrated with a ceramic-like material, comprising the following steps:
partial or complete coating or infiltration of a substrate with a solution of the aforementioned oligosilazane-containing condensation product,
evaporation of the solvent and curing of the oligosilazane-containing condensation product in the coated or infiltrated substrate,
as well as a substrate coated or infiltrated with a ceramic-like material obtained according to this process.